Synergistic Effect of ZIF-8 and Pt-Functionalized NiO/In2O3 Hollow Nanofibers for Highly Sensitive Detection of Formaldehyde

A rapid and accurate monitoring of hazardous formaldehyde (HCHO) gas is extremely essential for health protection. However, the high-power consumption and humidity interference still hinder the application of HCHO gas sensors. Hence, zeolitic imidazolate framework-8 (ZIF-8)-loaded Pt-NiO/In2O3 hollow nanofibers (ZPNiIn HNFs) were designed via the electrospinning technique followed by hydrothermal treatment, aiming to enable a synergistic advantage of the surface modification and the construction of a p-n heterostructure to improve the sensing performance of the HCHO gas sensor. The ZPNiIn HNF sensor has a response value of 52.8 to 100 ppm HCHO, a nearly 4-fold enhancement over a pristine In2O3 sensor, at a moderately low temperature of 180 °C, along with rapid response/recovery speed (8/17 s) and excellent humidity tolerance. These enhanced sensing properties can be attributed to the Pt catalysts boosting the catalytic activity, the p-n heterojunctions facilitating the chemical reaction, and the appropriate ZIF-8 loading providing a hydrophobic surface. Our research presents an effective sensing material design strategy for inspiring the development of cost-effective sensors for the accurate detection of indoor HCHO hazardous gas.


Introduction
Formaldehyde (HCHO) is regarded as a common indoor pollutant.Prolonged exposure to high levels of HCHO can lead to a variety of health problems [1][2][3].The World Health Organization (WHO) guidelines have set an exposure threshold value of 82 ppb for HCHO in indoor living areas [4].At present, metal oxide semiconductor (MOS)-based sensors are one of the effective methods for monitoring indoor HCHO with higher sensitivity and shorter reaction times [5,6].However, the high-power consumption of MOS sensors and the susceptibility of their sensitivity to ambient humidity are the main drawbacks that affect the reliability of gas sensors in practical applications [7].Thus, it is necessary to develop an MOS-based sensor with a lower operating temperature (<200 • C) and good anti-humidity for high-sensitivity HCHO detection.
The surface modification of MOS structures by noble metals (Pt, Ag, and Pd) with unique electronic states and chemical properties reduces the adsorption activation energy and promotes the electron transfer efficiency, thus enhancing the sensing capabilities of MOS [8].For instance, Liu et al. [9] synthesized Ag-SnO 2 nanoparticles, and the sensitivity reached 14.4 towards 10 ppm HCHO at 125 • C. In addition, p-n heterojunctions can integrate the physical and Nanomaterials 2024, 14, 841 2 of 12 chemical properties of p-and n-type MOS into one system [2,10,11].Nickel oxide (NiO), as a typical p-type metal oxide, can accelerate the catalytic decomposition of the target gas and promote the gas-sensing reaction on the surface [12,13], further improving the response/recovery characteristics and reducing the operating temperature of the gas sensor.Kumar et al. [14] prepared SnO 2 -decorated NiO nanoparticles and investigated the catalytic kinetics during the HCHO adsorption-desorption process.As mentioned above, the construction of hybrid materials combining noble metals and p-n heterojunctions is expected to realize the highly sensitive detection of HCHO gas at low temperatures.
It has been suggested that the surface modification of MOS materials can improve the water-resistance performance of gas sensors by ensuring minimal effects from humidity [15][16][17].Metal-organic frameworks (MOFs) featuring a high specific surface area and abundant porous structure can effectively avoid contact between water molecules and MOS-sensing materials [18].Meanwhile, gas molecules can diffuse through the pores of MOFs [19].Hence, MOF-based hydrophobic surface modifications have been found to enhance the anti-humidity performance of MOS-based gas sensors without reducing their sensitivity.
Zeolitic imidazolate framework-8 (ZIF-8), a well-studied MOF with a robust structure and highly porous properties, is composed of Zn 2+ ions coordinated with 2-methylimidazolate ligands, which has a wide range of applications in gas sensors.In addition, the hydrothermal technique is a facile and effective method for the synthesis of gas-sensing materials.Ferlazzo et al. [20] prepared samarium oxide (Sm 2 O 3 ) nanorods by a facile hydrothermal route for efficient detection of volatile organic compounds gas in the indoor environment.Jin et al. [21] constructed SnO 2 /SnS 2 n-n heterojunction anchored on rGO using a hydrothermal route for detection HCHO.
Herein, ZIF-8 loaded on the surface of Pt-NiO/In 2 O 3 hollow nanofibers (ZPNiIn HNFs) were synthesized by facile electrospinning technique combined with the subsequent hydrothermal method to serve as the high-performance HCHO gas-sensing materials.To evaluate the effect of p-n NiO/In 2 O 3 heterojunctions, ZIF-8, and noble metal Pt nanoparticles on sensor performance, HCHO-sensing measurements based on pristine In 2 O 3 , NiO/In 2 O 3 , ZIF-8@NiO/In 2 O 3 , and ZPNiIn sensors were carried out.The ZPNiIn sensor presents excellent HCHO-sensing properties with a high response value, high rapid response/recovery speed, and excellent anti-humidity compared with the other three sensors.The enhanced sensing mechanism of the ZPNiIn HNF-based sensor was further analyzed, which is related to the high catalytic activity of Pt, the formation of interfacial heterojunction, and the appropriate loading of ZIF-8 nanoparticles.

Chemicals
All the chemicals and reagents used in this work can be found in the Supporting File (Text S1).

Synthesis of Pristine In 2 O 3 and NiO/In 2 O 3 Hollow Nanofibers (NiIn HNFs)
NiIn HNFs were prepared with the one-step electrospinning method.In detail, 1 g of In(NO 3 ) 3 •4.5H 2 O and 0.2 g of Ni(NO 3 ) 2 •6H 2 O were dissolved in N,N-dimethylformamide (8 mL) and ethanol (5 mL) under vigorous stirring.Then, an electrospun precursor was formed by adding 1 g of PVP to the mixture and stirring for 6 h at room temperature.An electrostatic voltage of 14 kV was applied between the needle tip of a 10 mL syringe containing the above precursor.Finally, further calcination at 600 • C for 3 h in air with a heating rate of 5 • C per min −1 formed NiIn HNFs from the precursor fibers.This same process was used to prepare the pristine In 2 O 3 HNFs without Ni(NO 3 ) 2 •6H 2 O.
2.3.Synthesis of ZIF-8@NiO/In 2 O 3 (ZNiIn) and ZIF-8@Pt-NiO/In 2 O 3 (ZPNiIn) HNFs In the ZIF-8@Pt-NiO/In 2 O 3 (ZPNiIn) HNF synthesis process, 0.4 g of zinc oxide and 0.6 g of 2-methylimidazole were dissolved in methanol (30 mL), forming mixture A. Then, 0.05 g of as-prepared NiIn HNFs was suspended in methanol with 0.15 mL of 10 mg mL −1 H 2 PtCl 6 •6H 2 O solution.The prepared suspension was ultrasonically treated for 20 min; then, a reducing agent of 0.008 g of NaBH 4 was added dropwise, and the mixture was stirred to form mixture B. The above two solutions were then mixed and continually stirred to form a homogenous solution.Then, the solution was transferred to a Teflon reactor and heated to 120 • C for 3 h.We collected the precipitate and washed it with methanol after centrifugation.Finally, the obtained ZPNiIn HNFs were dried at 60 • C overnight.The ZNiIn HNFs were obtained with the same process without the addition of H 2 PtCl 6 •6H 2 O and NaBH 4 .

Characterizations
SEM (GeminiSEM 500, China) and TEM (JEOL JEM2100, Japan) were used to characterize the samples' morphology.X-ray diffraction (XRD) patterns were acquired on a PANalytical X'pert MPDPro (The Netherlands) using a Cu Kα radiation source (40 kV, 40 mA).The AXIS ULtrabld (UK) instrument was used to obtain X-ray photoelectron spectroscopy (XPS) using a monochromatic Al Kα radiation source (15 kV, 1486.6 eV).Fourier-transform infrared spectroscopy (Bruker Tensor 37 spectrometer, China) was used to determine surface functional groups and chemical bonds of samples.An HR800 Raman spectrometer (France) was used for Raman scattering, and excitation was carried out with 532 nm laser light.The specific surface area and pore sizes of the samples were detected with N 2 adsorption/desorption using an SSA-4300 (China) instrument.Sessile drops were measured using the optical contact angle meter (OCA25HTV, DataPhysics, China), where distilled water drops were placed on solid surfaces using a dispenser.

Gas Sensor Fabrication and Sensing Tests
Using drops of deionized water, the prepared materials were mix into a homogeneous paste.Then, this paste was coated uniformly on the ceramic tube with Au electrodes, further annealed in a muffle furnace, and annealed at 150 • C for 4 h.As shown in Figure S1, gas-sensing performance was evaluated using a WS-30A measurement system after aging the sensors for seven days in air [22].A static liquid-gas distribution method was used to prepare the different concentrations of tested gases.First, reference air from the cylinder was continuously introduced into the gas distribution chamber at a rate of 50 mL/min to replace the air in the gas distribution chamber for 10 min.After stopping the introduction of reference air, a certain volume Q of HCHO solution (37%) was collected using a microsyringe, injected into the evaporator in the gas distribution chamber, and heated to evaporation.Finally, the HCHO vapor is completely mixed with the reference air using a built-in fan.Specifically, acetone, benzene, toluene, methanol, ethanol, and formaldehyde vapors were obtained by evaporating acetone (≥99.5%),benzene (≥99.5%),toluene (≥99.5%),methanol (≥99.5%),ethanol (≥99.7%), and formaldehyde (37%) solution, respectively, and the methane gas was the standard gas.The volume Q can be obtained with the Equation (S1).The specific description is shown in Text S2.

Morphology Characterization and Phase Composition
The schematic illustration of the ZIF-8@Pt-NiO/In 2 O 3 HNFs is illustrated in Figure 1 [22].SEM images, as illustrated in Figure 2a-c, present that the obtained nanofibers of pristine In 2 O 3 , NiIn, ZNiIn, and ZPNiIn HNFs consist of continuous nanofibers that form web-like network structures (Figures 2a-c and S2).A clear hollow structure of NiIn nanofibers can be observed in Figure 2b (red circle in inset Figure 2b).Unlike the smooth surfaces of In 2 O 3 and NiIn HNFs, the ZPNiIn surface was uniformly loaded with ZIF-8 nanoparticles.The sharp contrast occurred between the dark edge and the pale interval in TEM images (Figure 2d,e) of ZIF-8@Pt-NiO/In 2 O 3 , confirming the hollow structure.Lattice fringes of 0.23, 0.29, and 0.21 nm correspond to the crystal planes of Pt (111) [23], In 2 O 3 (222) [24], and NiO (200) [10], respectively (Figure 2f).The EDS elemental mappings in Figure 2g reveal that In, Ni, O, Pt, Zn, and N elements evenly distribute in ZPNiIn HNFs, proving the formation of a heterojunction.The EDS spectrum shown in Figure S3 further presents that the percentages of In, Ni, O, Pt, Zn, and N are 22.68%, 6.54%, 68.22%, 0.48%, 1.39%, and 0.69%, respectively.
faces of In2O3 and NiIn HNFs, the ZPNiIn surface was uniformly loaded with ZIF-8 nanoparticles.The sharp contrast occurred between the dark edge and the pale interval in TEM images (Figure 2d,e) of ZIF-8@Pt-NiO/In2O3, confirming the hollow structure.Lattice fringes of 0.23, 0.29, and 0.21 nm correspond to the crystal planes of Pt (111) [23], In2O3 (222) [24], and NiO (200) [10], respectively (Figure 2f).The EDS elemental mappings in Figure 2g reveal that In, Ni, O, Pt, Zn, and N elements evenly distribute in ZPNiIn HNFs, proving the formation of a heterojunction.The EDS spectrum shown in Figure S3 further presents that the percentages of In, Ni, O, Pt, Zn, and N are 22.68%, 6.54%, 68.22%, 0.48%, 1.39%, and 0.69%, respectively.faces of In2O3 and NiIn HNFs, the ZPNiIn surface was uniformly loaded with ZIF-8 nano-particles.The sharp contrast occurred between the dark edge and the pale interval in TEM images (Figure 2d,e) of ZIF-8@Pt-NiO/In2O3, confirming the hollow structure.Lattice fringes of 0.23, 0.29, and 0.21 nm correspond to the crystal planes of Pt (111) [23], In2O3 (222) [24], and NiO (200) [10], respectively (Figure 2f).The EDS elemental mappings in Figure 2g reveal that In, Ni, O, Pt, Zn, and N elements evenly distribute in ZPNiIn HNFs, proving the formation of a heterojunction.The EDS spectrum shown in Figure S3 further presents that the percentages of In, Ni, O, Pt, Zn, and N are 22.68%, 6.54%, 68.22%, 0.48%, 1.39%, and 0.69%, respectively.To further investigate the crystal structure of the as-prepared samples, the Raman spectra were investigated.As shown in Figure 3c, the Raman spectrum of pristine In2O3 presents peaks located at 306, 496, and 629 cm −1 , which are ascribed to the δ (InO6), In-O-In, and υ(InO6), respectively [25].The peak of NiO at 495 cm −1 belongs to the optical mode of longitudinal vibration (LO).The peak located at 1074 cm −1 indicates the 2LO phonon vibration of NiO [30].Compared with the In2O3 and NiO, the Raman peaks of NiIn, ZNiIn, and ZPNiIn HNFs exhibit a slight blue shift due to the electron-phonon interaction generated by heterojunction formation, indicating that heterojunctions can modulate the chemical bonds and electrical properties [31].To further investigate the crystal structure of the as-prepared samples, the Raman spectra were investigated.As shown in Figure 3c, the Raman spectrum of pristine In 2 O 3 presents peaks located at 306, 496, and 629 cm −1 , which are ascribed to the δ (InO 6 ), In-O-In, and υ(InO 6 ), respectively [25].The peak of NiO at 495 cm −1 belongs to the optical mode of longitudinal vibration (LO).The peak located at 1074 cm −1 indicates the 2LO phonon vibration of NiO [30].Compared with the In 2 O 3 and NiO, the Raman peaks of NiIn, ZNiIn, and ZPNiIn HNFs exhibit a slight blue shift due to the electron-phonon interaction generated by heterojunction formation, indicating that heterojunctions can modulate the chemical bonds and electrical properties [31].
The nitrogen adsorption-desorption measurements of pristine In 2 O 3 and ZPNiIn HNFs were carried out in Figure 3d,e.For the pristine In 2 O 3 and ZPNiIn HNFs, the nitrogen adsorption/desorption isotherms show the typical IV isotherm with an H3 hysteretic loop, indicating the coexistence of micropores and mesopores (Figure 3d).Furthermore, the filling of ZIF-8 micropores increased ZPNiIn HNF sorption under low relative pressure [32].
The BET surface area of the ZPNiIn HNFs is 760.6 m 2 g −1 , which is a nearly 40-fold increase compared to that of pristine In 2 O 3 HNFs (19.1 m 2 g −1 ), demonstrating the enhanced gas adsorption performance of ZPNiIn HNFs.The average pore diameter of pristine In 2 O 3 and ZPNiIn HNFs are 7.9 nm and 23.9 nm, respectively.
The XPS spectra of samples were obtained to analyze the elemental composition and valence states.The peaks at 443.9-444.3eV and 451.5-451.9eV are found in all the samples corresponding to In 3d 5/2 and In 3d 3/2 , respectively, verifying the presence of In 3+ [33] (Figure 3f).For Zn 2p spectra, peaks corresponding to Zn 2p 3/2 and Zn 2p 1/2 of Zn 2+ at 1021.8 eV and 1044.7 eV are observed in ZNiIn and ZPNiIn HNFs due to the ZIF-8 loaded on the surface of NiO/In 2 O 3 HNFs (Figure 3g).The spectra of Ni 2p (Figure 3h) present two peaks of Ni 2p 1/2 (871.6 eV) and Ni 2p 3/2 (853.1 eV) for NiIn, ZNiIn, and ZPNiIn samples, indicating the existence of Ni 2+ [34].Figure 3i shows the O 1s spectra of pristine In 2 O 3 and ZPNiIn HNFs.The spectra of O 1s can be categorized as lattice oxygen (O L ), oxygen vacancies (O V ), and chemisorbed oxygen (O C ) [35].The area ratios of the oxygen fractions of samples are further listed in Table S1.The adsorption of air oxygen on the surface of the sensing materials results in the formation of O C , which can react with the analyzed gas [36].The contents of O C in pristine In 2 O 3 and ZPNiIn HNFs are 23.3% and 36.9%,demonstrating the ZIF-8 loaded and the heterojunction formation contribute to increasing the surface O C content.The significantly increased proportion of O C can facilitate the gas-sensing reaction and enhance the sensitivity.

Gas-Sensing Performance
To evaluate the effect of p-n heterojunctions, ZIF-8, and noble metal Pt nanoparticles on the sensor performance, HCHO-sensing measurements based on pristine In 2 O 3 , NiIn, ZNiIn, and ZPNiIn sensors were carried out.As depicted in Figure 4a, the responses of pristine In 2 O 3 , NiIn, ZNiIn, and ZPNiIn HNF sensors to HCHO (100 ppm) were investigated at the operating temperatures of 140-220 • C. The results of the four sensors indicate that response values are trending upward until they reach the maximum of 13.5 (In 2 O 3 ), 20.5 (NiIn), 32.6 (ZNiIn), and 52.8 (ZPNiIn) at 180 • C.Then, the responses decreased as the operating temperature increased from 180 to 220 • C.Among them, the ZPNiIn sensor exhibits an excellent response value, 3.9-fold higher than the In 2 O 3 sensor.The substantial enhancement in the HCHO response value can be ascribed to the synergistic impact of ZIF-8 and Pt modification and the construction of a heterojunction.Hence, the optimal operating temperature for the sensors is 180 • C. Figure S4 displays the baseline sensor resistance (R a ) in air at the operating temperatures of 140-220 • C. The results reveal that the R a of the four sensors decreases with increasing temperature, which is due to the characteristics of metal oxide semiconductors [37].Notably, the resistance of ZNiIn and ZPNiIn sensors dramatically increased, which was caused by the poor electric properties of ZIF-8 [38].
response of the ZPNiIn sensor to HCHO is significantly higher than to other interfering gases.Meanwhile, the sensitivity of the ZPNiIn sensor surpasses that of other sensors, demonstrating its excellent selectivity.The five consecutive cyclic response/recovery curves of the ZPNiIn sensor toward 100 ppm HCHO at 180 °C are shown in Figures 4f  and S7, and the response value of the sensors only fluctuates slightly with unchanged response/recovery times, suggesting the excellent reproducibility of the ZPNiIn sensor.Figure 4c reveals the gas response curves of sensors toward different HCHO concentrations (1-100 ppm) at 180 • C. It can be seen from the curve that the gas response values of sensors increase as the HCHO concentration increased.Among these four sensors, the ZPNiIn sensor presents the highest response values toward different concentrations of HCHO.Moreover, all sensors present good linearity relationship between HCHO concentrations and the response within the given concentration range (Figure 4d).The theoretic limits of detection (LODs) of the sensor can be calculated based on Equation (1) [39,40]: where RMS noise denotes the root mean square noise based on 50 experimental baselines in the air.The slope can be obtained in the concentration-response curve.After calculation, the LOD of the ZPNiIn sensor to HCHO gas is calculated as low as 46.7 ppb, while the LODs of pristine In 2 O 3 , NiIn, and ZNiIn are 258.2,122.3, and 63.8 ppb, respectively, implying the ZPNiIn sensor can be used to detect HCHO at lower ppb concentrations.Furthermore, the responses of four sensors toward different interfered volatile organic compound (VOC) gases (100 ppm) were also investigated at 180 • C (Figure 4e).The response of the ZPNiIn sensor to HCHO is significantly higher than to other interfering gases.Meanwhile, the sensitivity of the ZPNiIn sensor surpasses that of other sensors, demonstrating its excellent selectivity.The five consecutive cyclic response/recovery curves of the ZPNiIn sensor toward 100 ppm HCHO at 180 • C are shown in Figures 4f and S7, and the response value of the sensors only fluctuates slightly with unchanged response/recovery times, suggesting the excellent reproducibility of the ZPNiIn sensor.
The interference of humidity on pristine In 2 O 3 and ZPNiIn sensors' sensitivity toward 100 ppm HCHO was also evaluated as displayed in Figure 4g.With the increase in humidity from 27% to 70% RH, the HCHO response value of the ZPNiIn sensor is reduced by 14.2%, which is significantly superior to that of the pristine In 2 O 3 sensor (the response decreased by 58.7%).Meanwhile, the baseline resistances of both sensors are decreased with increasing humidity (Figure S6).Notably, the ZPNiIn sensor can still achieve a high response value to HCHO under 70% RH (R a /R g = 44.4),indicating the improved humidity tolerance of the sensor.The water contact angles between sensing materials and drops of water are shown in Figure 4h.The ZPNiIn has stronger hydrophobicity with the water contact angle of 34.2 • , which is bigger than pristine In 2 O 3 (19.9• ).The improved anti-humidity of the ZPNiIn sensor is mainly attributed to the surface modifications with ZIF-8 coating [41] and the functionalization of Pt noble metals [15,42].Based on Figure 4i, the sensitivity of the ZPNiIn sensor toward HCHO has barely fluctuated over 30 days, implying the significant long-term stability of the ZPNiIn sensor.To further estimate the sensing properties of the ZPNiIn sensor, some similar works on HCHO gas-sensing are summarized as listed in Table S2.The ZPNiIn sensor also exhibits a more outstanding sensitivity toward HCHO gas compared with those reported HCHO sensors.

Gas-Sensing Mechanism
During the adsorption and desorption of target gas molecules on the surface of the sensing material, MOS changes its electrical resistance to detect the gas [43,44].The obtained ZIF-8@Pt/NiO-In 2 O 3 presented n-type semiconducting gas-sensing characteristics.In the ambient atmosphere, the adsorbed oxygen on the sensing material surface is then ionized into O − 2 , O − , or O 2− , depending on when the operating temperature is 150 • C or less, between 150 and 400 • C, or more than 400 • C, respectively [45].The optimum temperature in our work is 180 • C. Thus, the adsorbed oxygen ions of this work are O − (Equation ( 3)).Once the HCHO gas has been introduced, it reacts with O − , resulting in the generation of CO 2 and electrons (Equation ( 4)); electrons are released again into the sensing material's conduction band.The process is expressed by Equations ( 2)-( 4) as follows: O 2 (ads) + e − → 2O − (ads) HCHO (gas) +2O − (ads) → CO 2 (gas) + H 2 O (gas) + 2e − (4) Figure 5 displays the schematic diagram of ZPNiIn for HCHO gas sensing.Sensors based on ZPNiIn are more sensitive to HCHO than those based on other materials.This could be explained as follows: On the one hand, heterojunctions increase oxygen molecule absorption and target gas detection.When these three components come in contact, since the work function of NiO (9.4 eV) [37] is greater than that of In2O3 and Pt, electrons will migrate from In2O3 and Pt to NiO until their Fermi energy levels reach equilibrium, and the Schottky barrier formed at the contact interfaces, as shown in Figure 5a,b.The aggregation of electrons on NiO increases its concentration and improves its ability to capture oxygen, thereby promoting oxygen adsorption.This results in the superior performance of the isopropanol sensor since the resistance changes considerably during the sensing process.
On the other hand, the chemical sensitization of Pt NPs on the surface of NiO/In2O3 composites is the main reason for the improvement of the gas-sensing capability [46,47].The Pt NPs facilitate the dissociation of O2 molecules into adsorbed oxygen (O − ) through the "spillover effect", increasing the concentration of adsorbed oxygen (Figure 5c).Therefore, ZPNiIn possessed more surface-adsorbed oxygen, which is favorable to promote the active sites for enhancing the gas-sensing properties [48].The characterization results of XPS also confirmed that the adsorbed oxygen content of ZPNiIn increased (Table S1).
Moreover, the coated ZIF-8 NPs also play a significant role in the improvement of the sensing capabilities.From the results of the N2 adsorption-desorption measurement, the specific surface area of the ZPNiIn HNFs is increased 40-fold due to the load of ZIF-8.This can increase the number of active sites available for the adsorption of the target HCHO gas during sensing [38,49].For practical applications, water vapor is partially repellent from the hydrophobic ZIF-8 and does not interfere with sensing performance under humidity conditions [50].In addition, gas adsorption and diffusion are facilitated by the inner and outer surfaces of ZPNiIn hollow structures, which provide ample active sites for gas reactions [51].

Conclusions
In order to develop a high-performance HCHO sensor for indoor pollutant detection, the ZIF-8 loaded Pt/NiO-In2O3 HNFs were synthesized using the electrospinning method On the one hand, heterojunctions increase oxygen molecule absorption and target gas detection.When these three components come in contact, since the work function of NiO (9.4 eV) [37] is greater than that of In 2 O 3 and Pt, electrons will migrate from In 2 O 3 and Pt to NiO until their Fermi energy levels reach equilibrium, and the Schottky barrier formed at the contact interfaces, as shown in Figure 5a,b.The aggregation of electrons on NiO increases its concentration and improves its ability to capture oxygen, thereby promoting oxygen adsorption.This results in the superior performance of the isopropanol sensor since the resistance changes considerably during the sensing process.
On the other hand, the chemical sensitization of Pt NPs on the surface of NiO/In 2 O 3 composites is the main reason for the improvement of the gas-sensing capability [46,47].The Pt NPs facilitate the dissociation of O 2 molecules into adsorbed oxygen (O − ) through the "spillover effect", increasing the concentration of adsorbed oxygen (Figure 5c).Therefore, ZPNiIn possessed more surface-adsorbed oxygen, which is favorable to promote the active sites for enhancing the gas-sensing properties [48].The characterization results of XPS also confirmed that the adsorbed oxygen content of ZPNiIn increased (Table S1).
Moreover, the coated ZIF-8 NPs also play a significant role in the improvement of the sensing capabilities.From the results of the N 2 adsorption-desorption measurement, the specific surface area of the ZPNiIn HNFs is increased 40-fold due to the load of ZIF-8.This can increase the number of active sites available for the adsorption of the target HCHO gas during sensing [38,49].For practical applications, water vapor is partially repellent from the hydrophobic ZIF-8 and does not interfere with sensing performance under humidity conditions [50].In addition, gas adsorption and diffusion are facilitated by the inner and outer surfaces of ZPNiIn hollow structures, which provide ample active sites for gas reactions [51].

Conclusions
In order to develop a high-performance HCHO sensor for indoor pollutant detection, the ZIF-8 loaded Pt/NiO-In 2 O 3 HNFs were synthesized using the electrospinning method combined with hydrothermal techniques.The ZPNiIn sensor exhibits an enhanced response (R a /R g ) of 52.8 towards 100 ppm HCHO with a fast response (8 s) and recovery (17 s) time at 180 • C.Moreover, the sensor shows excellent selectivity and good anti-humidity.The improved gas-sensing properties can be attributed to the heterojunctions between components, the catalytic and sensitization effects of Pt NPs, and the coated ZIF-8.This study presents an effective strategy to design and fabricate high-performance HCHO sensors.

Figure 4 .
Figure 4. Sensing property measurements for pristine In 2 O 3 , NiIn, ZNiIn, and ZPNiIn HNFs at 180 • C. (a) Response toward HCHO (100 ppm) under different operating temperatures; (b) response and recovery characteristics of sensor exposure to HCHO (100 ppm); (c) response curves of sensors to HCHO from 1 to 100 ppm concentrations; (d) linearity between the concentration of sensors and the response value; (e) selectivity of sensors toward 100 ppm of various VOC gas; (f) repeatability of the ZPNiIn sensor to HCHO (100 ppm); (g) dynamic response curves of the pristine In 2 O 3 and ZPNiIn sensors to HCHO (100 ppm) under different relative humidities (RHs); (h) water contact angles of the pristine In 2 O 3 and ZPNiIn HNF-sensing materials' surface; (i) long-term stability of ZPNiIn sensor towards HCHO (100 ppm).

Figures
Figures 4b and S5 depict the real-time response and recovery curves of four sensors toward 100 ppm HCHO at 180 • C. The response/recovery time of pristine In 2 O 3 , NiIn, ZNiIn, and ZPNiIn sensors are 15/34 s, 10/12 s, 10/14 s, and 8/17 s, respectively.Compared to pristine In 2 O 3 , the ZPNiIn sensor offers a 2-fold increase in response/recovery speed.Figure 4c reveals the gas response curves of sensors toward different HCHO concentrations (1-100 ppm) at 180 • C.It can be seen from the curve that the gas response values of sensors increase as the HCHO concentration increased.Among these four sensors, the ZPNiIn sensor presents the highest response values toward different concentrations of HCHO.Moreover, all sensors present good linearity relationship between HCHO concentrations and the response within the given concentration range (Figure4d).The theoretic limits of detection (LODs) of the sensor can be calculated based on Equation (1)[39,40]:

9 of 12 13 Figure 5 .
Figure 5. (a,b) The energy level diagram and (c) the schematic sensing mechanism of ZPNiIn HNFs.

Figure 5 .
Figure 5. (a,b) The energy level diagram and (c) the schematic sensing mechanism of ZPNiIn HNFs.